1. Field of the Invention
The subject invention pertains to the art of radar surveillance systems and more particularly to a radar surveillance system which provides target information in three dimensions.
2. Description of the Prior Art
Ground to air radar surveillance systems which require only range and azimuth information may be realized by continuously rotating a fan beam antenna which provides a narrow azimuth beam width and a broad elevation beam width. These systems are capable of locating the range and azimuth coordinates of the detected target to an accuracy which is determined by the transmitted pulse width and the antenna azimuth beam width. The antenna's broad elevation beam width precludes the determination of a target's elevation coordinate to within any useful tolerance. To provide reasonable three coordinate information (3-D) modifications to the system and antennas are required.
One modification to fan beam surveillance systems to accomplish 3-D target location reduces the observation of elevation angle to a double observation of azimuth angle. In this system, a first azimuth observation is made by vertical fan beam and the second observation is made by a beam slanted to form a 45.degree. V-shaped trough with the first. The target's elevation is then determined from the measured range and the azimuth rotation of the antenna between the first and second azimuth observations.
Other systems have been devised to provide 3-D coverage without the complexity of the V-beam system. In these systems, the fan beam antenna is replaced with a pencil beam antenna which has narrow azimuth and elevation beam width characteristics. Scanning this pencil beam in azimuth and elevation provides the capability of determining the elevation coordinate. This two-dimensional scanning is accomplished by scanning the antenna in elevation while continuously rotating it in azimuth, thus creating an over-all spiral scan. To provide the elevation information with these systems, the azimuthal rotation rate of the system must be decreased by a factor that is a function of the number of elevation beam positions desired, thus significantly reducing the over-all data rate of the system.
Elevation information may be obtained without a data rate reduction by stacking a number of pencil beams in elevation, to each of which there is a corresponding receiver and noting the beam in which the target is detected. In these systems, each beam may be the antenna pattern of an independent monostatic radar system or a separate broad coverage fan beam may be utilized to illuminate the area with each of the stacked beams utilized for reception only. Another method for obtaining a 3-D target information is to employ a multibeam electronically scanned antenna wherein each beam may be independently controlled. These systems, however, provide only limited azimuthal sector coverage and three or four may be required to give the elevation coverage desired over a full 360.degree. azimuth coverage.
Surveillance systems heretofore described are extremely complex and generally inefficiently utilize the available radiation power. It is the object of the present invention to provide a 3-D system with a complete 360.degree. azimuth coverage, that exhibits high radiation efficiency with a minimum of system complexity.